To achieve increased memory density, phase change memories can be used. The memory cells of a phase change memory can each have multiple states that correspond to different resistances. A memory cell of a phase change memory is effectively a variable resistor, which can be formed of chalcogenide glass, for example. To write to a phase change memory cell, the cell is heated, which melts the material, which is then cooled. The rate of cooling controls the crystal size of the resulting solid material, which controls the resistance. A relatively slow cooling rate promotes crystallization, which leads to reduced resistance of the memory cell.
A phase change memory cell can be a multilevel memory cell that can represent more than two states. The number of states is represented by a corresponding number of resistance levels to which the memory cell can be programmed.